In our local mtns in So Cal we have rubber boas and Calif mtn kings. The habitat and seasons when you are most likely to find them lends one to believe that moisture is key.

On the other hand, I have since figured out that too much moisture on the surface soil is not conducive to seeing the animals.

Also, there is a condition known as "zonata disease" that can afflict captive zonata. I bring this up since the zonata and rubber boas often share the same habitat. I've always thought that too little moisture was the key. But, a friend who had a few hatchlings in different cages had the specimens in the mesh-top cages not get the condition, while the more enclosed and more humid shoebox specimens did. He has a snake room that has more ambient humidity than say my house.

Perhaps the answer is that these animals inhabit areas where they can find shelter that does not dessicate them, but is not damp either, at least most of the time. Perhaps these animals frequent a damp area when they need it, but stay away when they don't. Which makes your occasional soak idea a good one.